Portable electric impact tool

ABSTRACT

A portable electric impact drill, wrench or an analogous power tool wherein the motor shaft drives the intermediate shaft of a two-speed transmission which rotates the tool spindle. The transmission has a first gear rigid with the intermediate shaft and meshing with a second gear which is rotatable on the tool spindle but can drive the spindle by way of an overrunning clutch when the spindle is to rotate at the lower of two speeds. A third gear which is rigid with the spindle meshes with a fourth gear which is rotatable and axially movable on the intermediate shaft. The latter has a transverse bore for two balls which are biased apart by a helical spring to bear against the internal surface of the fourth gear. This fourth gear has an axial bore including a larger-diameter portion which receives portions of the balls in one axial position of the fourth gear whereby the intermediate shaft can rotate relative to the fourth gear to drive the spindle by way of the first and second gears and the overrunning clutch. A smaller-diameter portion of the axial bore in the fourth gear communicates with two axially parallel grooves which receive portions of the balls in a second axial position of the fourth gear whereby the latter drives the spindle by way of the third gear. The overrunning clutch then enables the spindle to rotate relative to the second gear.

United States Patent [191 Gotsch et a1.

May 7, 1974 PORTABLE ELECTRIC IMPACT TOOL [75] Inventors: Dieter Gotsch, Plattenhardt; Reinhard I-Iahner, Stuttgart; Manfred Kim, Stuttgart; l-Ians Lederer, Stuttgart, all of Germany [73] Assignee: Robert Bosch GmbH, Stuttgart,

Germany [22] Filed: Nov. 1, 1972 [21] Appl. No.: 302,751

[30] Foreign Application Priority Data Nov. 2, 1971 Germany 7141263 [52] US. Cl 74/343, 74/368, 74/371 [51] Int. Cl. F16h 3/32 [58] Field of Search 74/343, 342, 371, 368

[56] References Cited UNITED STATES PATENTS 850,257 4/1907 Schabinger 74/371 1,081,092 12/1913 Chalmers 74/371 2,557,799 6/1951 Salmivuori 74/368 X 1,564,667 12/1925 Guild 74/371 X 3,133,450 S/1964 Riley, Jr. et al. 74/342 3,178,955 4/1965 Enders et al 74/342 X FOREIGN PATENTS OR APPLICATIONS 1,957,234 5/1971 Germany 74/371 a J 1:] O 27 0 L 37 L 1Q 20 l 117 Primary Examiner-Benjamin W. Wyche Assistant ExaminerJ. Reep Attorney, Agent, or FirmMichael S. Striker [57] ABSTRACT A portable electric impact drill, wrench or an analogous power tool wherein the motor shaft drives the intermediate shaft of a two-speed transmission which rotates the tool spindle. The transmission has a first gear rigid with the intermediate shaft. and meshing with a second gear which is rotatable'on the tool spindle but can drive the spindle by way of an overrunning clutch when the spindle is to rotate at the lower of two speeds. A third gear which is rigid with the spindle meshes with a fourth gear which is rotatable and axially movable on the intermediate shaft. The latter has a transverse bore for two balls which are biased apart by a helical spring to bear against the internal surface of the fourth gear. This fourth gear has an axial bore including a larger-diameter portion which receives portions of the balls in one axial position of the fourth gear whereby the intermediate shaft can rotate relative to the fourth gear to drive the spindle by way of the first and second gears and the overrunning clutch.

1 A smaller-diameter portion of the axial bore in the fourth gear communicates with two axially parallel grooves which receive portions of the balls in a second axial position of the fourth gear whereby the latter drives the spindle by way of the third gear. The overrunning clutch then enables the spindle to rotate relative to the second gear.

PORTABLE ELECTRIC IMPACT TOOL BACKGROUND OF THE INVENTION ments in portable power tools wherein the tool spindle can be driven by a multi-speed transmission.

It is already known to provide a portable electric power tool with a multi-speed transmission which can drive the tool spindle at several speeds. When driven at the lowest of two or more speeds, the tool spindle receives torque by way of an overrunning clutch which is deactivated in response to shifting into a higher gear. The shifting is effected by engaging or disengaging one or more claw couplings which form part of the transmission. A drawback of such transmissions is that the power tool must be disengaged from a workpiece whenever the user wishes to shift from a higher speed to a lower speed or vice versa. This is undesirable for a number of reasons, mainly because the drilling, screw driving, nut or bolt applying or removing, tapping or any other operation which is being performed by the tool must be interrupted for each speed change. The transmission can be shifted into a lower or higher gear only when the motor is started or is in the process of coming to a halt and while the tool is disengaged from the workpiece unless, by pure coincidence, the claws of a previously disengaged coupling happen to be in a predetermined position for engagement while the motor is at a standstill.

SUMMARY OF THE INVENTION An object of the invention is to provide a power tool, particularly a portable electric impact wrench or the like, with a novel and improved transmission which can be actuated to change the speed of the tool spindle at any stage of operation of the power tool, i.e., while the motor is idle, while the motor is idling with the tool disengaged from a workpiece or while the tool engages with the workpiece, for example, to drill or tap a hole.

Another object of the invention is to provide the transmission with a novel and improved coupling which allows for changes in the speed of the tool spindle at the will of the user, irrespective of the position of the tool relative to the workpiece and irrespective of the condition of the motor.

A further object of the invention is to provide a novel and improved two-speed transmission for use in portable electric impact tools.

An additional object of the invention is to provide a portable power tool wherein the speed of the tool spindle can be changed with the exertion of a relatively small force, without any danger to the user, and without risking damage to the parts of the transmission and /or to the prime mover of the power tool.

The invention is embodied in a power tool, particularly in a portable impact drill, wrench or the like, and preferably in a portable power tool whose prime mover is an electric motor. The power tool comprises a rotary output element which constitutes the shaft of the aforementioned prime mover, a rotary tool spindle which is mounted in the housing of the power tool in parallelism with the output element, and a multi-speed transmission (preferably a two-speed geartransmission) which is interposed between the output element and the tool spindle and includes a rotary torque transmitting member (such as an intermediate shaft) drivenby the output element, a rotary torque receiving member (such as a gear which is rotatable on the intermediate shaft) arranged to drive the tool spindle, and a coupling which is actuatable to transmit motion from the torque transmitting member to the torque receiving member. Inaccordance with a feature of the invention, the coupling comprises at least one spherical motion transmitting element movably mounted in but rotatable with one of the transmission members and resilient means for biasing the spherical element against the other transmission member. The coupling preferably comprises at least two spring-biased spherical motion transmitting elements.

The transmission preferably further comprises overrunning clutch means interposed between a third rotary member of the transmission and the tool spindle to allow the latter to rotate with the torque receiving member when the torque receiving member is driven by the torque transmitting member.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved power tool itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary longitudinal vertical sectional view of a portable electric impact drill which, by way of example, embodies the invention;

FIG. 2 is a fragmentary longitudinal vertical sectional view in a plane parallel to the plane of FIG. 1;

FIG. 3 is a transverse vertical sectional view as seen in the direction of arrows from the line III-III of FIG. 1; and

FIG. 4 is an end elevational view of a gear on the tool spindle of the power tool shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 and 3, there is shown a portable electric power tool which can be used as'an impact wrench, drill, screwdriver, screw remover or for other purposes. It is assumed that the tool is an impact drill. The housing of the power tool comprises two shells l, 2 consisting of synthetic plastic material and forming an integral pistol grip handle including the shell portions 3 and 4. The prime mover 5 in the housing including the shells l, 2 is a series-wound electric collector motor which comprises a stator package 6 mounted in the shell 1 together with a rear bearing 7 for the output shaft 13. The pistol grip handle 3, 4 is fonned with an opening for a cable 8 which is surrounded by a protective sheath 9 to prevent excessive flexing. The conductors 10 of the cable 9 are connected with the electric motor 5 by way of an antiinterference capacitor 11 and a depressible trigger switch 12.

The output shaft 13 is connected with the rotor package 15 by way of an insulating sleeve 14. A fan 16 is mounted on the output shaft 13 in front of the insulating sleeve 14 and behind a suitable seal or partition 17 which subdivides the interior of the housing into a rear chamber or motor chamber 18 and a front chamber or transmission chamber 19. The chamber 19 accommodates a bearing block or bridge 20 consisting of sinter metal and secured to the shell 1 by two screws 21 or analogous fasteners. The bridge 20 has an opening for an antifriction ball bearing 22 whose inner race surrounds the front portion of the output shaft 13. The

foremost portion 23 of the output shaft 13 extends forwardly beyond the antifriction bearing 22 and constitutes a pinion. The bridge 20 further receives a rotary selector pin 24 whose axis is normal to the axis of the output shaft 13 and whichis located at a level above the output shaft, i.e., the pistol grip handle 3, 4 is located at one side and the selector pin 24 is located at the opposite side of the shaft 13. The pin 24 can be rotated by way of a knob 25 consisting of synthetic plastic material and being accessible at the upper side of the motor housing. The selector pin 24 is a metallic cylinder having an axially parallel flat 26 located behind the rear end portion of a rotary tool spindle 28 which is parallel with the output shaft 13. The rear end portion of the tool spindle 28 is-rotatable and axially movable in a friction hearing or sleeve bearing 27 mounted in the bridge 20. When the selector pin 24 assumes the angular position of FIG. 1 (i.e., when the flat 26 faces away from the rear end portion of the tool spindle 28), the cylindrical peripheral surface of the pin 24 serves as an abutment for a sphere 29 which is received in a recess or socket provided therefor in the rear end face of the tool spindle. The front end portion of the tool spindle 28 is formed with an external thread 30 mating with the internal thread of a chuck 31 which can removably receive and hold a suitable drilling tool, screw driving tool or the like. The rear end face of the chuck 31 can abut against an enlarged intermediate portion or collar 32 of the tool spindle 28.

The front end portion of the housing including the shells 1 and 2 constitutes a cylindrical neck 33 which receives an antifriction ball bearing 34 for the tool spindle 28. The antifriction bearing 34 is located behind the collar-32, and the tool spindle 28 is rotatable and axially movable in the inner race of this bearing. The front end position of the tool spindle 28 is determined by a split ring 36 which is recessed into a peripheral groove of the tool spindle behind the bearing 34 and abuts against the inner race of the bearing 34 when the tool spindle assumes its front end position. A helical spring 35 reacts against the inner race of the bearing 34 and bears against the rear end face of the collar 32 to normally hold the tool spindle 28 in the foremost position in which the split ring 36 abuts against the inner race of the bearing 34.

The tool spindle 28 is moved axially from the illustrated front end position when the tip of the tool is applied against a workpiece (not shown) and the operator exerts pressure against the pistol grip handle 3, 4 to move the entire power tool (save for the tool spindle 28 and chuck 31) forwardly. The extent of rearward axial movement of the tool spindle 28 depends on the angular position of the selector pin 24, i.e., whether the sphere 29 can abut against the cylindrical peripheral surface or against the flat 26 of the selector pin 24. In the illustrated angular position of the pin 24, the tool spindle 28 has only sufficient freedom of a reduced axial movement for a drilling operation without impacts. The power tool will operate as an impact wrench, drill or screwdriver when the user turns the sesstqr p 24 through about grees ,sqathatths sphere 29 can be moved into abutment with the flat 26 to thus provide more room for axial movements of the tool spindle 28.

The tool spindle 28 is rigidly connected with a spur gear 37 which forms part of the transmission in the chamber 19 and is located in front of the friction bearing 27. The rear end face of the gear 37 is provided with an annulus of projections 38 facing a similar annulus of projections 39 on the front surface of the bridge 20. When the flat 26 of the selector pin 24 faces forwardly (i.e., toward the rear end portion of the tool spindle 28), the projections 38 can travel along the projections 39 to insure that the tool in the chuck 31 receives a series of axial impulses to promote its penetration into the material or a workpiece. It will be noted that the transmission of axial impulses to the tool which is carried by the tool spindle 28 can be started and terminated by the simple expedient of changing the angumunicate with the bore 41. Each of the grooves 42 is surrounded by a surface including a cylindrical first portion extending around about three-fourths of the respective groove 42 and a flat or substantially flat portion which constitutes a gradual transition between the first surface portion and the surface surrounding the adjacent portion of the axial bore 41. In other words, the flat surface portion is substantially tangential to that portion of the tool spindle 28 which is received in the bore 41. The just mentioned portion of the tool spindle 28 in the bore 41 of the gear 40 is formed with a diametrically extending transverse bore 43 which receives two balls 44 having radii which are only slightly smaller than the radii of curvature of the aforementioned cylindrical first portions of surfaces surrounding the grooves 42. The transverse bore 43'of the tool spindle 28 further receives a helical spring 45 which tends to move the balls 44 away from each other, i.e., against the internal surface of the gear 40. The central portion of the gear 40 with its grooves 42 and the parts mounted in the transverse bore 43 of the tool spindle 28 constitute an overrunning clutch which allows the tool spindle 28 to rotate relative to the gear 40 when the speed of the tool spindle exceeds the speed of the last mentioned gear.

The transmission in the chamber 19 also includes a torque transmitting intermediate shaft 46 best shown in FIG. 2. The intermediate shaft 46 is parallel to the output shaft 13 and tool spindle 28 and its rear end portion extends into a friction bearing or sleeve bearing 47 mounted in the bridge 20. The front end portion of the intermediate shaft 46 is mounted in a sleeve bearing 48 provided in theshell 1. A gear 49 is fixedly secured to the intermediate shaft 46 in front of the sleeve bearing 47 to mesh with the pinion 23 of the output shaft 13. A torque receiving gear 50 is rotatably and axially movably mounted on the intermediate shaft 46 in front of the gear 49. The gear 50 meshes with the gear 37 on the tool spindle 28 and comprises a forwardly extending hub 51 having a cylindrical external surface 52 provided with a circumferential groove 53. The smallerdiameter portion 54 of the axial bore of the gear 50 (see also FIG. 3) communicates with two axially parallel grooves 55 each of which extends along about onethird of the axial length of the gear 50. The grooves 55 are adjacent to the rear axial end of the gear 50, i.e., adjacent to that end which is nearer to the motor 5. Each of the grooves 55 is surrounded by a substantially semicylindrical internal surface of the gear 50. The diameter of that portion of the axial bore of the gear 50 which is located in front of the grooves 55 equals the diameter of the surrounded portion of the intermediate shaft 46 plus the combined depth of the grooves 55. This larger-diameter portion of the axial bore of the gear 50 is denoted by the reference character 56; The gear 50 is further formed with suitably inclined surfaces 57 providing a gradual transition between the surface surrounding the bore portion 56 on the one hand and the surfaces surrounding the grooves 55 and the surface surrounding the smaller-diameter portion 54 of the axial bore on the other hand.

That portion of the intermediate shaft 46 which is surrounded by the gear 50 is formed with a diametrically extending transverse bore 66 receiving two spherical motion transmitting elements or balls 67 and a helical spring 68 which is located between and tends to move the balls 67 away from each other. Depending on the axial position of the gear 50, the balls 67 are biased against the surface surrounding the bore portion 56 or against the surface surrounding the bore portion 54 (namely, that portion of the axial bore of the gear 50 which communicates with the grooves 55). In the latter axial position of the gear 50, the balls 67 penetrate into the oncoming grooves 55 in response to a relatively short angular movement of the intermediate shaft 46 relative to the gear 50, or vice versa. The front end portion of the intermediate shaft 46 (behind the sleeve bearing 48) is provided with or connected to a small gear 58 meshing with the gear on the tool spindle 28.

As shown in FIG. 3, the shell 2 of the housing is provided with an inwardly extending sleeve 59 which is adjacent to the gear on the intermediate shaft 46, i.e., the sleeve 59 is located in the chamber 19. The cylindrical bore or passage 60 of the sleeve 59 receives a rotary shifter 61 which consists of synthetic plastic material and includes a handgrip portion or knob 62 accessible at the outer side of the shell 2. The knob 62 preferably seals the outer end of the passage 60 against the entry of foreign matter. The inner end portion of the shifter 61 is slotted and extends slightly beyond the inner end of the sleeve 59, for example, through a distance of one millimeter. The inwardly projecting end portion of the shifter 61 is formed with a small collar 63 which cooperates with the sleeve 59 to hold the shifter against extraction from the shell 2. The shifter 61 is provided with an eccentric axially parallel bore 64 for a metallic pin or post 65 which extends into the circumferential groove 53 in the peripheral surface 52 of the hub 51. Thus, if the user decides to turn the knob 62, the pin or post 65 moves the gear 50 axially of the intermediate shaft 46 to thereby change the position of the gear 50 relative to the balls 67 in the transverse bore 66 of the shaft 46.

The operation:

In order to start the motor 5, the user depresses the switch 12 whereby the output shaft 13 begins to rotate in a clockwise direction, as viewed from the handle 3, 4 toward the tool spindle 28. The pinion 23 is integral with the output shaft 13 and rotates the gear 49 and the intermediate shaft 46 in a counterclockwise direction whereby the gear 58 of the intermediate shaft 46 rotates the gear 40 in a clockwise direction. If the gear 50 is held by the pin 65 in its rear end position (nearer to the motor 5 as shown in FIG. 2), the intermediate shaft 46 is free to rotate in the gear 50. The tool spindle 28 is then driven in a clockwise direction from the pinion 23 by way of the gear 49, intermediate shaft 46, gear 58, gear 40 and balls 44 of the overrunning clutch. The speed of the tool spindle 28 is relatively low.

If the user wishes the tool spindle 28 to rotate at the higher of two speeds, the knob 62 is rotated so that the pin 65 of the shifter 61 moves the gear 50 forwardly, namely, in a direction to the left, as viewed in FIG. 2. This causes the balls 67 of the coupling between the shaft 46 and gear 50 to immediately enter the grooves 55 in the gear 50 without any further compression of the spring 68, or the spring 68 is stressed while the balls 67 move toward each other to engage the cylindrical surface surrounding the smaller-diameter portion 54' of the axial bore of the gear 50. A relatively small angular displacement (not more than of the intermediate shaft 46 suffices to cause the balls 67 to penetrate into the oncoming" grooves 55 to thus establish a motion transmitting connection between the shaft 46 and the gear 50. The pinion 23 continues to rotate the interme diate shaft 46 counterclockwise by way of the gear 49, and the gear 50 (which is now rotated by the shaft 46 and the balls 67) rotates the gear 37 and the tool spindle-28 in a clockwise direction. The speed of the tool spindle 28 increases so that the latter can rotate relative to the slowly rotatinggear 40 which continues to be driven by the gear 58 on the intermediate shaft 46. The overrunning clutch including the balls 44 allows the tool spindle 28 to rotate with reference to the gear 40. Such rotation of the tool spindle 28 at the higher speed results in depression of balls 44 into the transverse bore 43 against the opposition of the spring 45.

The gear 50 can be shifted by the knob 62 while the motor 5 is at a standstill as well as while the motor 5 drives the tool spindle 28, irrespective of whether the tool in the chuck 31 engages with or is disengaged from a workpiece. This is rendered possible by the provision of the coupling including the balls 67 and by the provision of the clutch including the balls 44. The clutch allows the tool spindle 28 to rotate relative to the gear 40 while the latter continues to receive torque from the intermediate shaft 46 by way of the gear 58. The feature that the transmission can shift from a low speed to a high speed or vice versa while the power tool is in use brings about substantial savings in time because the user can manipulate the knob 62 whenever he deems it necessary to change the rotational speed of the tool. The changing of tool spindle speed in actual use of the power tool does not result in damage to the parts of the transmission because the aforedescribed configuration of the gear 50 allows the balls 67 to move from engagement with the surface surrounding the bore portion 56 into the grooves 55 or vice versa at any time, i.e., irrespective of whether or not the intermediate shaft 46 is driven by the pinion 23 of the output shaft 13.

The coupling means between the torque-transmitting intermediate shaft 46 and the torque-receiving gear 50 of the transmission in the chamber 19 may comprise more than two spherical elements 67 or a single spherical element. For example, the intermediate shaft 46 can be provided with two axially spaced transverse bores 66 each of which receives two spherical elements 67 with a resilient element 68 therebetween. If the axes of the two bores are located in a common plane, the inv ternal surface surrounding the smaller-diameter portion 54 of the axial bore in the gear 50 must be provided with only two grooves 55; otherwise, the gear 50 will be provided with four discrete axially parallel grooves 55, one for each of the four spherical elements.

It is further clear that the transmission in the chamber 19 may be a threeor four-speed transmission; the transmission then comprises two or more couplings each of which includes at least one spring-biased spherical motion transmitting element disposed between a torque transmitting member (such as the intermediate shaft 46) and a torque receiving member (such as the gear 50) of the transmission.

The intermediate shaft 46 can be driven by the rotary output element of a prime mover other than an electric motor, for example, by the output shaft of a pneumatic motor.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a power tool, particularly in a portable electric impact tool, a combination comprising prime mover said torque transmitting member to said torque receiving member and including at least one spherical motion transmitting element mounted in one of said members and resilient means for biasing said spherical element against the other of said members, a second gear rotatably mounted on said spindle, a third gear meshing with said second gear and receiving torque from said intermediate shaft, overrunning clutch means interposed between said spindle and said second gear to allow said spindle to rotate relative to said second gear when said spindle is driven by said intermediate shaft by way of said coupling and said first gear, and a fourth gear rigid with said spindle and meshing with said first gear.

2. A combination as defined in claim 1, wherein said coupling comprises a plurality of spring-biased spherical motion transmitting elements.

3. A combination as defined in claim 1, wherein said intermediate shaft has a transverse bore surrounded by said first gear and said coupling comprises two spherical elements movably received in said bore, said resilient means comprising spring means installed in said bore between said spherical elements and arranged to bias said spherical elements radially outwardly against said gear.

4. A combination as defined in claim 3, wherein said first gear is movable between first and second axial positions and has an axial bore including .a largerdiameter portion receiving portions of said spherical elements in one axial position of said gear and a smaller-diameter portion surrounding said transverse bore in the other axial position of said first gear, said first gear further having an internal surface surrounding said smaller-diameter portion of said axial bore and provided with two axially parallel grooves receiving portions of said spherical elements in said other axial position of said first gear whereby said intermediate shaft can rotate said first gear by way of said spherical elements.

5. A combination as defined in claim 1, wherein said first gear is movable axially of said intermediate shaft between a first position in which said first gear receives torque from said intermediate shaft by way of said coupling and a second position in which said intermediate shaft is free to rotate relative to said first gear, said first gear having a hub provided with an external circumferential groove and further comprising rotary shifter means provided with an eccentric pin extending into said groove to move said firstgear between said first and second positions in response to rotation of said shifter means.

I. i I! 

1. In a power tool, particularly in a portable electric impact tool, a combination comprising prime mover means including a rotary output element; a rotary spindle; and a two-speed transmission interposed between said output element and said spindle, said transmission including a rotary torque transmitting member constituting an intermediate shaft driven by said output element, a rotary torque receiving member constituting a first gear arranged to drive said spindle and axially movably and rotatably surrounding said intermediate shaft, a coupling actuatable to transmit motion from said torque transmitting member to said torque receiving member and including at least one spherical motion transmitting element mounted in one of said members and resilient means for biasing said spherical element against the other of said members, a second gear rotatably mounted on said spindle, a third gear meshing with said second gear and receiving torque from said intermediate shaft, overrunning clutch means interposed between said spindle and said second gear to allow said spindle to rotate relative to said second gear when said spindle is driven by said intermediate shaft by way of said coupling and said first gear, and a fourth gear rigid with said spindle and meshing with said first gear.
 2. A combination as defined in claim 1, wherein said coupling comprises a plurality of spring-biased spherical motion transmitting elements.
 3. A combination as defined in claim 1, wherein said intermediate shaft has a transverse bore surrounded by said first gear and said coupling comprises two spherical elements movably received in said bore, said resilient means comprising spring means installed in said bore between said spherical elements and arranged to bias said spherical elements radially outwardly against said gear.
 4. A combination as defined in claim 3, wherein said first gear is movable between first and second axial positions and has an axial bore including a larger-diameter portion receiving portions of said spherical elements in one axial position of said gear and a smaller-diameter portion surrounding said transverse bore in the other axial position of said first gear, said first gear further having an internal surface surrounding said smaller-diameter portion of said axial bore and provided with two axially parallel grooves receiving portions of said spherical elements in said other axial position of said first gear whereby said intermediate shaft can rotate said first gear by way of said spherical elements.
 5. A combination as defined in claim 1, wherein said first gear is movable axially of said intermediate shaft between a first position in which said first gear receives torque from said intermediate shaft by way of said coupling and a second position in which said intermediate shaft is free to rotate relative to said first gear, said first gear having a hub provided with an external circumferential groove and further comprising rotary shifter means provided with an eccentric pin extending into said groove to move said first gear between said first and Second positions in response to rotation of said shifter means. 